A plurality of files are put into a single archive file with the file names and file hierarchy information retained, and the initial files, file names, and file hierarchy are restored from the archive file. FIG. 1A shows an example in which an encoder 10 collectively compresses a plurality of files and a folder in a folder (folder name “folder1”) in directory C in an environment where Windows (registered trademark) is used as an OS (operating system) environment, into a single archive file ARC (file name “C:/folder1.archive”). The folder “folder1” contains a text file (file name “file1.text”) and a folder “folder 2”. The folder “folder2” contains three files: file2*JPN1*EUC.wav (hereinafter *JPN1* indicates a Japanese kana and/or kanji character string that means “the Japanese language”), file3*JPN1*UTF-8.aiff, and file4*JPN1*SJIS.mp4. The whole of these folders and files in the file hierarchy will be referred to as a file set FS.
FIG. 1B illustrates that a decoder 20 decodes the archive file ARC generated as shown in FIG. 1A and expands it to the initial file set FS as shown in FIG. 1A. Known tools for compressing and restoring this type of hierarchical file set include GZIP and WinZIP. In the hierarchical file set illustrated as an example in FIGS. 1A and 1B, the files file 2, file 3, and file 4 contain music data or video data, and the file names are recorded in a play list held in the file 1 text file. If the file set FS is input to an application program such as a software music player, the software music player references the play list in file 1 and successively selects music data files that can be played by the software music player from the folder 2 folder and plays them.